In recent years, electrical devices, such as mobile phones, laptop computers, electronic notebooks, digital cameras, and game machines, generally have audio capabilities. A sound emitter such as a speaker or a buzzer, or a sound receiver such as a microphone, is disposed inside a housing of an electrical device with audio capabilities. An opening is provided in the housing of the electrical device at a position corresponding to the sound emitter or the sound receiver. Sounds are transmitted through the opening. In addition, these electrical devices are required to have waterproofness. To meet this requirement, it is known to use a waterproof sound-transmitting membrane to cover the opening of a housing of an electrical device with audio capabilities.
Patent Literature 1 and Patent Literature 2 propose the use of a plastic membrane, such as a porous polytetrafluoroethylene (hereinafter sometimes referred to as “PTFE”) membrane, as a waterproof sound-transmitting membrane. Patent Literature 1 and Patent Literature 2 also propose a waterproof sound-transmitting membrane including a plastic membrane and further a support placed on the membrane to provide adequate strength and good workability to the waterproof sound-transmitting membrane. As examples of the support, nets of thermoplastic resins such as polyolefin and polyester are mentioned.
Patent Literature 3 discloses a porous PTFE membrane (waterproof sound-transmitting membrane) obtained by fusing a thermoplastic resin net to a longitudinally-stretched PTFE sheet and then stretching the PTFE sheet with the net attached thereto in the transverse direction. This production method is intended to improve the sound-transmitting characteristics and water resistance of the waterproof sound-transmitting membrane.